1. Field of the Invention
The invention relates to ferritic stainless steel alloys. More particularly, the invention relates to an iron-chromium-aluminum alloy having rare earth and magnesium and/or calcium additions.
2. Description of Related Art
U.S. Pat. No. 4,414,023 discloses a ferritic stainless steel alloy which can be used as a catalytic substrate. The alloy includes, by weight, 8.0-25.0% Cr, 3.0-8.0% Al, at least 0.002% and up to 0.05% Ce, La, Nd, and/or Pr with the total of all rare earths up to 0.06%, up to 4.0% Si, 0.06% to 1.0% Mn and normal steelmaking impurities of less than 0.050% C, less than 0.050% N, less than 0.020% O, less than 0.040% P, less than 0.030% S, less than 0.050% Cu, less than 0.050% Ni, and the sum of Ca and Mg less than 0.005%, the remainder being Fe. The '023 patent discloses that the steel can be heat treated to form an aluminum oxide surface which is adherent and provides for thermal cyclic oxidation resistance. The '023 patent further discloses that known processes for producing alumina whiskers on ion-chromium-aluminum alloys to further increase the surface area and provide more effective catalyst retention on the surface for improving catalyst efficiency include either:
1. Producing a thin strip with a heavily cold-worked surface by removing the strip from a solid log through a machining process called "peeling" and subjecting the strip to 870.degree.-930.degree. C. in air, as disclosed in United Kingdom Patent Application GB 2,063,723 A; or PA1 2. Using a thin strip produced by conventional hot and cold rolling, preconditioning the surface by heating for a short time to temperatures of about 900.degree. C. in an essentially oxygen-free inert atmosphere (less than 0.1% O.sub.2), and after cooling to room temperature performing a whisker growing heat treatment in air for longer periods of time at about 925.degree. C.